Matthew "Matt" Devlin is DS Ronnie Brooks' younger and more impulsive partner, friend, and surrogate son. He sincerely looks up to Brooks but takes a more aggressive and black-and-white stand on their cases. Matt is from an Irish-Catholic family in Kilburn. Cheeky and charming, Matt's policing instinct is second to none. In equal parts a force to be reckoned with and a good-looking playboy, Matt brings energy and optimism to the partnership.

Devlin has a romanticized idea of policing and absolutely loves his job and believes in what he does. Despite rejecting the "absolutism" of his upbringing, he still has a black-and-white view of the law: people are either innocent or guilty. Hardworking, efficient and thorough, Matt has a traditional view of the law: but he'll also push past the rule of procedure when he feels his cause is just. He has little sympathy for people who commit crimes, whatever their circumstances might have been.

Inseparable now for five years, Matt and Ronnie are mischievous when they're together: each recognizes the twinkle in the other's eye and they are known at the station as Morecambe and Wise.

In "Deal", after Kaden Blake is proved innocent of the murder of Lia Brown, Matt Devlin escorts Kaden to the child care unit, but sees someone in the distance wearing a mask, the masked man draws a pistol and aims at Kaden Blake, but Matt's final heroic deed is literally jumping in the line of the bullets while the episode goes to slow motion. Devlin is shot twice in the back, the bullets go through Devlin's back and spurt out of both sides of his chest, and Devlin is still in mid air, but the slow motion sequence stops and Devlin is flung onto the pavement and hits his back on the concrete floor, Ronnie runs over and puts his jacket over Matt to stop the bleeding, Matt looks around and sees Kaden, who is in shock, and Matt also sees fellow Detective Sergeant Riley get shot in the chest by three more bullets. Ronnie demands an ambulance and he looks at Matt and yells for someone to get an ambulance, Matt then looks away from Riley, who is dead and back at Ronnie, who is trying desperately to keep him alive, but as the screen goes black, Matt's eyes close,and a "TO BE CONTINUED…" sign appears and the episode ends.Matt is revealed to have died of his wounds in part 2 of the episode, "Survivor's Guilt," which features Brooks and new detective Sam Casey investigating Devlin's death. But after Sam Casey's few episodes of working with Brooks, Matt reappears, but it is unknown if these are flashback episodes. After series 2 Matt has a new screen,in the intro to his Prior Episodes, his intro screen shows him looking at something at a crime scene and running. But Matt's new intro shows him in a car park underneath a building and running shouting "RONNIE!!!!!!", A scene from Honour Bound from Series 2.